The present invention relates generally to inverter systems employing inverter circuits of the type having a commutating capacitor and more particularly to systems employing such inverter circuits in conjunction with circuitry to guard against currents which become too large to be properly and dependably commutated.
There are many types of inverter systems. A number of these systems employ capacitors which are charged to a voltage value and that capacitor voltage is used during the next cycle to commutate an associated controlled rectifier. One such inverter system is the Autosequentially Commutated Current Source Inverter, sometimes more simply referred to as the Controlled Current Inverter. In this type of inverter, the value of the voltage on the capacitor is directly proportional to current which flows in the inverter circuit (i.e., the load current) prior to the last commutation of a rectifier of the group with which it is associated. Thus, the amount of current which can be commutated at any particular instant in time is directly proportional to the amount of current which was commutated by the particular rectifier group the last time. This being true, the amount of current rise which can be permitted between successive commutations is limited.
In many applications, the amount of current change between commutations creates no serious problem. For example, in a low performance motor drive where only slow changes in speed are required or when the load changes are gradual, the control may be designed to limit the rate of current rise and no problem will be experienced because of excess currents. In high performance drives, however, where rapid changes are required and thus high rates in change of currents are a necessity, the problems become more complex because the maximum permissible change in current between successive commutations is still limited by the design of the system. In addition, since many of these high performance applications require a variable frequency output, the rate of current change cannot be maintained constant without severely limiting the dynamic performance of the drive. One solution to the commutation problem which is sometimes proposed is to increase the size of the commutating capacitors. While the use of larger capacitors can substantially alleviate the problem of commutation failure at low frequencies, the top speed of the drive may be unacceptably limited for other reasons. Thus, solutions such as strictly limiting the rate of current rise and using larger capacitors are sometimes not acceptable.